Compression Check on removed motor

[copeland3300]

Hey all,

I wanted to do a compression check on my motor that's been removed from the bike. I know I need a battery, but I don't know where to connect it to turn the engine. Any thoughts would be great!

[CatatonicBug]

I'd start with the starter leads. Assuming you don't have carbs on them with a fuel source, that is going to be the only source of motion for the engine that could be battery-powered.

[bikerphil]

The starter is under a plastic cover in the center of the engine behind the head. If you remove the 3 bolts holding the cover down you will see the positive lead for the starter on the rear of the starter. Hook your positive jumper cable to that somehow. Don't forget to put the bolts back in without the cover attached to secure the starter. Hook your negative jumper cable to the engine case somewhere that is not painted and it should crank.

[copeland3300]

Awesome! Thanks for the help

[copeland3300]

Also, what values should I be looking for?

[bikerphil]

142 psi + or - 10% with throttle held wide open is what the service manual says.

[copeland3300]

Alright, thanks again

[crazy steve]

If all the plugs are pulled and the carbs are off, don't be surprised to get much higher numbers. Main thing is if they're even, and if they're not, check valve lash. Maladjusted valves can have a big effect on compression....

[copeland3300]

Thanks for that added info. I'm going to be working on this guy today, so ill update when I know more. Thanks to you both!

[MPittma100]

Compression Test

If one or more of the cylinders compression is low, you should run a cylinder leakage test to find where the loss is going. Typical leaks are by the valves or rings. Leakage percentage sould be no more than 15 percent.

The only other place for leakage is the head gasket which is not common on these engines.

MP

[CaptonZap]

For those that are not familiar with the check, here is a primer.


http://www.engineersedge.com/power_t...sion_check.htm


The gauge set can be constructed, but it is a lot easier to find someone with one, and have them do it, or loan you the gauges, if they trust you that well.
The results will give you a good idea as to the health of that cylinder/piston set.
A little browse through Google, under "differential pressure check" will lead you to sites that explain the process. Or try http://www.tcmlink.com/pdf2/SB03-3.pdf
CZ

[copeland3300]

Hmmm, ok, well here's the deal:

I finally got all the new gaskets I needed to install installed.
I filled with 4 liters of 15w40, no oil filter (I know I have to install one soon, it in the mail), got a new battery and charged it up.

I got the engine to turn over, and I was reading about 45-60 psi in each piston.

Now there could be a few things I think:

improper installation of the gauge
the tester having too long of a hose
the engine having an issue
etc.

But they did all check out around the same-ish pressure.

I'm going to go at it again tomorrow, and check on the other engine before I do anything else. That should give me a good baseline for the tester. I think.

In any case, if it's in fact reading correct, I'll get a leak checker as suggested and go from there.

Thanks all!

[natemoen]

Wait till you get a filter so you don't pump all the settled sludge in the engine into the bearings!

I am sorry but that is just stupid, especially if the filter is on it's way.

[CaptonZap]

"Getting the engine to turn over" conjures a picture of an engine that is struggling to rotate. If you used a low battery, or had a cold engine, or had the other three plugs installed, or had a weak starter, or had some mechanical internal drag which slowed the rotation of the engine down, or the engine has been setting for a few years, you could expect to have low readings. And the readings you got are low. The faster the engine rotates while testing, the higher the readings.
For that reason, some people suggest that the engine should be warmed up before testing. Obviously, with a non running engine, that is difficult.
However, you can get a little more information about the condition of the bores by pouring some light weight oil, like ATF, into the bores, turn the engine over a few revolutions, to spread the oil around the rings, and take another reading. Use a fully charged battery, such as the one in your car. If the readings come up about the same percentage per cylinder, chances are that the rings are OK, and just need to be run to regain their sealing properties.
If the readings do not improve, you need to find out which of the valves are leaking, and that will require a differential pressure checker. The fact that the readings are close, indicates that it is probably the rings, since the chances of all the valves being out of adjustment the same amount is slim. And if the engine has been setting for a long time, and you did not put any oil in the cylinders, which would leave the rings with no oil to seal them, chances are that you do not have a problem.
If you can get it running, you can check compression after a few hours run time, and see if it improves.
But do adjust the valves before trying to start the engine.
CZ

[copeland3300]

I know that the engine has been sitting for at least 2 years without being run, so it's probably accurate that it may need to be run for a bit before i'll get a decent reading.

When I had the cylinder heads off of the bike a few months ago, I did put some WD-40 into the cylinders and rotated them a few revolutions. I didn't notice much, if any, of it disappearing, but maybe my desire for a good working engine is coloring my memory.

In either case, I'm going to test the cylinders today and I'll post an update. BTW the filter came as well, so I'll be using that as well.